1. Field of the Invention
The present invention relates to data communication. In particular, the present invention relates to a signal modulation method of high spectral efficiency suitable for use in data communication without requiring modulation of a carrier signal.
2. Discussion of the Relevant Art
Signal constellations needed for M-ary phase-shift key (PSK) modulations (M>2) and quadrature amplitude modulation (QAM) are not known in a carrier-less communication system, such as impulse radio and some ultra-wideband (UWB) systems. In M-ary PSK modulation and QAM, phase and amplitude (or signed amplitude) are both available for encoding data. Using both phase and amplitude increases spectral efficiency and data rate. For example, consider a binary PSK (BPSK) signal that uses only constellation points ±A, and a quaternary PSK (QPSK) signal that uses only constellation points ±A and ±jA, where j is √{square root over (−1)}. The BPSK constellation points have phases of 0 and 180 degrees, and the QPSK constellation points have phases of 0, 180, and ±90 degrees. If the channel imposes an additive white Gaussian noise in the received signal, the BPSK and QPSK signals would both require the same signal-to-noise-ratio (SNR) per bit to achieve the same bit error rate (BER), even though the QPSK signal conveys twice as many bits over the same time duration and frequency bandwidth. Thus, the QPSK signal provides a higher spectral efficiency than a BPSK signal. An M-ary QAM signal has similar advantages over an M-ary PAM signal.
Exploiting both amplitude and phase of a signal requires generating both in-phase and quadrature signals at the transmitter. A quadrature signal is conventionally defined with respect to a modulated carrier signal that is 90 degrees out of phase (i.e., in quadrature) with a carrier used for the in-phase signal. In a conventional system, an explicit carrier signal at a prescribed frequency, together with mixing and frequency conversion techniques, can be used to generate in-phase and quadrature signals. However, neither a carrier signal, nor the signal mixing or frequency conversion techniques, is available in a carrier-less data communication system, such as UWB signals.
Similarly, an M-ary PSK modulation (M>2) phase-shifts a carrier. In a carrier-less system which, by definition, has no carrier to phase-shift, phase-shift keying is unknown in the prior art.